www.elsevier.com/locate/ica Inorganica Chimica Acta 326 (2001) 47 – 52
Aminoxyl radical substituted uranyl compounds Nathalie Daro a, Philippe Guionneau a,1, Ste´phane Golhen b,1, Daniel Chasseau a, Lahce`ne Ouahab b, Jean-Pascal Sutter a,* a
Laboratoire des Sciences Mole´culaires, Institut de Chimie de la Matie`re Condense´e de Bordeaux UPR CNRS N° 9048, F-33608 Pessac, France b Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR 6511, Institut de Chimie de Rennes, Uni6ersite´ de Rennes 1, Campus Beaulieu, F-35042 Rennes cedex, France Received 15 June 2001; accepted 24 July 2001 This paper is dedicated to the memory of Professor Olivier Kahn
Abstract A series of adducts formed by association of the uranyl ion, UO22 + , with either nitronyl nitroxide substituted pyridine and triazole ligands or their imino nitroxide counterpart are described. The stability of the radical unit in the vicinity of the U centre appeared to be dependent on whether this unit is directly involved or not in the coordination to the metal ion. The crystal structures of both types of compounds have been determined by X-ray diffraction. The temperature dependence of the magnetic behaviours in the solid state for the paramagnetic compounds revealed substantial exchange interaction among the spin carriers in the crystal. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nitronyl nitroxide; Uranyl; f-Element; Uranium; Molecular magnetism
1. Introduction Coordination compounds of lanthanide ions involving a second spin carrier such as paramagnetic ions or free radicals have been attracting much interest in recent years. Particular attention is devoted to the understanding of the mechanism of the exchange interaction occurring in such compounds. Several compounds, comprising transition metal ions [1 –6] or organic radical derivatives [7 – 12], have been designed for this purpose. The situation is much less advanced as far as the actinide ions are concerned. Recently two derivatives for which 5f2 U(IV) ions interact with either Cu(II) [13] or Mn(II) [14] have been reported. Ferromagnetic interactions were found for the former trinuclear compound whereas for the latter 3D-network a * Corresponding author. Tel.: + 33-557-962 544; fax: +33-556842 649. E-mail address:
[email protected] (J.-P. Sutter). 1 Authors for enquiries relating to the crystallographic studies.
paramagnetic behaviour was observed over the investigated temperature range. We now considered compounds for which a U ion would be surrounded by organic radicals acting as ligands. In the present report we describe a series of compounds obtained by reacting the uranyl ion, UO22 + , with either nitronyl nitroxide or the corresponding imino nitroxide derivatives. Although U(VI) is diamagnetic and will not be exchange coupled with its ligands, the aim of this study was to address several questions. First, will an aminoxyl free radical be chemically stable in the vicinity of a strongly Lewis-acid and oxophilic centre such as U? Second, considering the rather diffuse 5f orbitals as compared to the f orbitals of the lanthanide ions, how efficient is the U ion in a superexchange process involving two spin carriers of its coordination sphere? Third, might the uranyl ion be envisaged as a platform for arranging organic radicals in well-defined arrays leading to long-range intermolecular interactions?
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 5 9 3 - X
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2. Results and discussion
2.1. Synthesis and structural characterisation The paramagnetic ligands considered in this study are nitronyl nitroxide (i.e. 4%,4%,5%,5%-tetramethylimidazoline 1%-oxyl 3-oxide) substituted triazole and pyridine
derivatives and their imino nitroxide (i.e. 4%,4%,5%,5%-tetramethylimidazoline 1%-oxyl) counterparts. The numbering scheme adopted for the ligands and uranyl compounds is given in Chart 1. Two types of compounds were obtained depending on the ligand involved (Eq. (1)).
(1)
Chart 1.
Fig. 1. Molecular structure (ORTEP) for UO2(NO3)2(Nit-TRZ)2 (1) (H atoms are omitted for clarity). Selected bond lengths (A, ) and angles (°): UN1, 2.527(5); UO3, 1.756(2); UO4, 2.490(4); UO5, 2.502(4); N4O1, 1.278(3); N5O2, 1.270(3); C5N4, 1.343(4); C5N5, 1.354(4) A, . Torsion angle: N4N5N2N3, 52.2°. Symmetry transformation used to generate equivalent atoms: i: 1 − x, 1− y, − z.
The reaction with Imino-TRZ as well as the ortho-substituted pyridine derivatives leads to compounds 2 and 3, respectively, for which one ligand is coordinated to the UO2 centre. For all other derivatives a stoichiometry of two ligands per U was found. A dramatic difference in the magnetic behaviour of the two types of products was also observed. Compounds 1, 4–6 are paramagnetic with a magnetic moment at 300 K in agreement with two S= 1/2 spin per molecule (vide infra), whereas compounds 2 and 3 are diamagnetic. The crystal structures of compounds 1, 3 and 5 were solved by X-ray diffraction on single crystal. The structure of compound 1, UO2(NO3)2(Nit-TRZ)2, reveals that two paramagnetic ligands are linked to the UO2 centre by the means of the triazole rings (Fig. 1). They occupy trans positions in the equatorial plane, which is completed by the h2-coordination of the nitrato anions. The coordination of the ligand through the N-atom in position 1 of the triazole, N1, is rather unusual; this ligand is known to form N,O-chelate interactions with metal ions [8,11]. The observed coordination isomer might be related to steric considerations within compound 1. As a consequence, the radical moieties point outwards. The molecular arrangement in the crystal leads to a rather close packing of the nitronyl nitroxide units (Fig. 2). Distances as short as 3.158(4) A, (O1···O1*) and 3.251(3) A, (O1···N4*) are found between two adjacent groups. The small range of intermolecular distances in the NO/ON parallelogram indicates that a good overlap exists. The second NO group of each unit of such pairs is again located close to the NO of a neighbouring pair (O2···O2*, 3.206(6) A, ) but without NO/ON overlap (Fig. 3). Considering
N. Daro et al. / Inorganica Chimica Acta 326 (2001) 47–52
Fig. 2. View of the crystal packing for UO2(NO3)2(Nit-TRZ)2 (1) along c (the NO3 anions are not shown).
Fig. 3. Details of the intermolecular arrangement of the nitronyl nitroxide units for UO2(NO3)2(Nit-TRZ)2 (1). Selected distances (A, ): O1···O1*, 3.158(4); O1···N4*, 3.252(4); N4···N4*, 3.801(4); O1···C5*, 3.540(4); O2···O2*, 3.206(4); O2···N5*, 4.215(4).
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The molecular structure of compound 5, UO2(NO3)2(p-Nit-Py)2, is very similar to that of compound 1 but with p-Nit-Py ligands (Fig. 4). The crystal packing for this compound consists of a succession of organic layers, formed by the paramagnetic ligands, and inorganic layers of UO2(NO3)2 moieties (Fig. 5). In these 2-D arrays of radical units the shortest NO/NO separations are found for O2···O2* with, alternatively, 3.616(3) and 3.705(3) A, , the related O2···N3* distances are 4.133(3) and 3.659(3) A, , respectively. For O1 the shortest distance to a neighbouring radical unit is found to C6 (3.824 A, ), the C-atom joining the NO groups of the nitronyl nitroxide units, and N2 (4.006(3) A, ), the O1···O1* separation being 4.157(3) A, . The presence in aminoxyl derivatives of a functional group able to act as a coligand has been shown to enhance the coordination behaviour of the radical moiety to a metal centre. This is especially true if the ligand can form a chelate to the metal. In the ortho substituted pyridine derivatives, o-Nit-Py and o-Imino-Py, the radical moieties and the pyridyl group are in a favourable situation to have the molecule functioning as a chelating ligand. Indeed the o-Nit-Py derivative leads to compound 3 in which a single pyridine ligand interacts as a N,O-chelate to the uranyl centre (Fig. 6). However, the nitronyl nitroxide no longer exists in the adduct. In the course of the reaction, the radical unit is reduced and losses the O-atom of one of the NO groups and a H-atom is found on the corresponding N-atom. The presence of this hydrogen in the vicinity of N12 is clearly established by the X-ray data. Moreover, the bond lengths C14N10 (1.312(9) A, ) and C14N12 (1.324(9) A, ) indicate that the CN double bond of the five member ring is delocalised over the two CN bonds, suggesting that the positive charge of the zwitterionic ligand is shared by the two N-atoms. It can be noticed that the same compound 3 is also the product of the reaction of the imino nitroxide substituted pyridine derivative with the uranyl salt. There is good evidence that compound 2, formed by reaction with the Imino-TRZ radical, has a molecular structure closely related to that of compound 3. Indeed,
Fig. 4. Molecular structure (ORTEP) for UO2(NO3)2(p-Nit-Py)2 (5) (the H atoms are omitted for clarity). Selected bond lengths (A, ) and angles (°): UN1, 2.551(3); UO3, 2.503(3); UO4, 2.479(3); UO6, 1.753(3); O1N2, 1.274(3); O2N3, 1.268(3); C6N2, 1.349(3); C6N3, 1.346(3) A, . Torsion angle: N2N3C5C3, 29.2°.
the short NO/NO contacts between the spin carriers, the arrays of nitronyl nitroxide units may be described as pseudo-pairs with weaker interactions between the dimers.
Fig. 5. View of the crystal packing for UO2(NO3)2(p-Nit-Py)2 (5) along c. Selected distances: O1···O1*, 4.157(3); O2···O2*, 3.616(3) A, .
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interaction as for the ortho-pyridine derivatives, then the final ligand of the adduct is a reduced radical. It is likely that this process is induced by the metal centre and involves solvent molecules, nevertheless it remains puzzling. This overall reduction is not observed for the other radical ligands, which lead to the paramagnetic compounds 1, 4–6 almost quantitatively.
2.2. Magnetic properties
Fig. 6. Molecular structure (ORTEP) for UO2(NO3)2(o-Himino-Py) (3) (the CH2Cl2 solvate is not shown and the H atoms, except H121, are omitted). Selected bond lengths (A, ) and angles (°): UN3, 2.627(5); UO11, 2.297(5); UO24, 1.761(5); UO22, 1.761(5); N10O11, 1.349(5); C14N10, 1.312(5); C14N12, 1.324(5); UONO2, 2.503 – 2.545(5); O11UN3, 68.54(5); O22UO24, 178.11(5).
Fig. 7. Temperature dependence of M and MT for compound 1. The solid line represents the calculated best fit for J= − 53.4 90.4 cm − 1 and zJ% = − 3.0 9 0.8 cm − 1.
2 contains a single triazole derivative and is diamagnetic. This suggests that at an early stage of the reaction the Imino-TRZ has a chelate type coordination to the UO2 centre, whereas the Nit-TRZ is found to have h1-N-bonding to the metal ion. The chemical stability of the radical units observed for these U(VI) compounds is in strong contrast with the behaviour known, i.e. chemical compatibility, for related lanthanide derivatives. It appears that the stability of the radical in the vicinity of the U centre depends very much on its direct coordination, or not, to the metal ion. If the coordination of the nitronyl nitroxide or the imino nitroxide unit is forced by a chelate
The solid state temperature dependence of the molar magnetic susceptibility, M, for compounds 1–6 was investigated on a polycrystalline sample with a SQUID susceptometer in the temperature range 2–300 K, with an applied field of 1000 Oe. Whereas compounds 2 and 3 are diamagnetic, for compounds 1,4 –6 the observed behaviour indicates that antiferromagnetic interactions among the spin carriers tend to cancel the magnetic moment of the aminoxyl radicals. The plot of M versus T and MT versus T for compound 1 are shown in Fig. 7. The occurrence of rather strong antiferromagnetic interactions is demonstrated by a maximum of the M curve observed at 50 K. This is also reflected by the value of MT at 300 K, 0.70 cm3 K mol − 1, a value below the one expected for two S= 1/2 spin in a Curie regime. The characteristic feature of the M versus T curve is observed as well for the other compounds with the maximum at 11, 3 and 12 K for 4, 5 and 6, respectively. It is very unlikely that the observed magnetic behaviours have their origin in intramolecular exchange interactions, but result from intermolecular interactions. The X-ray analysis for compound 1 revealed that the radical units are arranged in pairs, which should contribute significantly to the magnetic interactions (Fig. 3). For this reason, the magnetic data for 1 have been analysed using a dimer-model (H= − JS1·S2), the interaction among the pairs, zJ%, was considered in the mean-field approximation. Least-square fitting to the experimental data led to J= −53.49 0.4 cm − 1 and zJ%= − 3.090.8 cm − 1 (g was fixed to 2.00). This rather strong exchange parameter for a ‘‘through space’’ interaction confirms the good overlap of the SOMO of the radical units within a dimer. The crystal packing for compound 5 (Fig. 5) suggests that several exchange pathways may be operative between neighbouring molecules which could result in a 2-D correlation. Therefore no attempt was made to analyse the experimental data by a theoretical model. Nevertheless, the aforementioned maximum observed in M versus T for compound 5, as well as for compounds 4 and 6, is an indication of the overall intermolecular antiferromagnetic interaction occurring among the spin carriers.
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3. Concluding remarks The results gathered in this study show that aminoxyl substituted U(VI) can be obtained provided the radical unit is not forced to bind the metal centre in the course of the reaction. This limitation hindered the access to a compound suitable for an investigation of a superexchange process involving the U(VI) ion. Finally, for all paramagnetic compounds a substantial intermolecular exchange interaction is found between the aminoxyl unit in the crystal suggesting that uranyl adducts might be an interesting possibility to organise organic radicals in the condensed matter.
4. Experimental The free radicals Nit-TRZ [15], and pyridine derivatives were prepared according to procedures described in the literature [16,17]. Commercial reagents and solvents were used as received.
4.1. 3 -(4 %,4 %,5 %,5 %-Tetramethylimidazoline 1 %-oxyl) -4,5 -dimethyl triazole (Imino-TRZ) Nit-TRZ (0.75 g, 3 mmol) was added to a suspension of NaNO2 (0.52 g, 7.5 mmol) and CH3COOH (1.8 ml, 30 mmol) in CH2Cl2 (40 ml). The reaction mixture was refluxed for a few minutes until a bright orange solution was obtained. The suspension was filtered off and the solution neutralised with a saturated NaHCO3/H2O solution (30 ml). After separation and extraction of the H2O solution with CH2Cl2 the organic layer was dried over Na2SO4 and concentrated to ca. 10 ml. By layering with hexane, Imino-TRZ was obtained as an orange crystalline compound (0.60 g, Y = 85%). Anal. Calc. for C11H18N5O: C, 55.91; H, 7.68; N, 29.64. Found: C, 55.77; H, 7.93; N, 29.46%. Melting Point: 131– 132 °C. IR spectrum (KBr, main peaks cm − 1): 3520 (m), 2278 (m), 3294 (w), 2982 (m), 2930 (w), 1670 (m), 1522 (s), 1438 (s), 1380(m), 1370 (m), 1258 (m), 1148 (m), 738 (m).
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Compound 2, orange crystals: Anal. Calc. for C11H19N7O9U+ 1/2C2H5OH: C, 22.02; H, 3.39; N, 14.98. Found: C, 21.87; H, 3.45; N, 14.85%. IR (KBr, main peaks cm − 1): 3425 (m), 3138 (m), 2996 (w), 1520 (s), 1384 (s), 1295 (s), 1216 (m), 1133 (m), 930 (s). Compound 4, deep blue–green crystals: Anal. Calc. for C24H32N8O12U: C, 33.42; H, 3.74; N, 12.99. Found: C, 33.51; H, 3.90; N, 12.60%. IR (KBr, main peaks cm − 1): 2986 (m), 1598 (m), 1538 (s), 1520 (s), 1453 (s), 1384 (s), 1322 (s), 1277 (s), 1030 (m), 937 (s). Compound 5, deep green crystals: Anal. Calc. for C24H32N8O12U: C, 33.42; H, 3.74; N, 12.99. Found: C, 33.12; H, 3.83; N, 12.89%. IR (KBr, main peaks cm − 1): 2989 (w), 1612 (m), 1534 (s), 1401 (m), 1382 (s), 1259 (s), 1020 (m), 934 (s). Compound 6, orange crystals: Anal. Calc. for C24H32N8O10U: C, 34.70; H, 3.88; N, 13.49. Found: C, 34.62; H, 4.00; N, 13.30%. IR (KBr, main peaks cm − 1): 2980 (w), 1617 (m), 1534 (s), 1421 (m), 1384 (s), 1521 (s), 1020 (m), 934 (s).
4.3. Compound 3 UO2(NO3)2 and either o-Nit-Py or o-Imino-Py (1 eq.) were mixed in absolute EtOH and the solution was layered with CH2Cl2. Compound 3 was obtained as orange square crystals within a few days. Anal. Calc. for C12H17N5O9U+ 1/2CH2Cl2: C, 22.89; H, 2.77; N, 10.68. Found: C, 22.96; H, 2.80; N, 10.55%. IR (KBr, main peaks cm − 1): 3296 (m), 2982 (w), 1588 (w), 1523 (s), 1466 (m), 1384 (s), 1290 (s), 1126 (m), 933 (s).
4.4. X-ray diffraction Selected experimental and crystal data are given in Table 1. X-ray diffraction data collection for compounds 1, 3 and 5 were run on an Enraf–Nonius CAD-4 diffractometer using a point detector. A semiempirical correction of absorption based on c scans of three axial reflections was applied. The crystal structures were solved and refined using the SHELX97 package [18]. For compounds 3 and 5, hydrogen atoms were located on Difference Fourier maps and refined isotropically.
4.2. Adduct synthesis: compound 1 A solution of UO2(NO3)2 (100 mg, 0.2 mmol) in absolute EtOH was layered with a solution of Nit-TRZ (100 mg, 0.4 mmol) in absolute EtOH. The deep blue crystals (140 mg) formed by slow mixing of the solutions were collected after 1 week. Anal. Calc. for C22H36N12O12U: C, 29.40; H, 4.04; N, 18.70. Found: C, 29.59; H, 4.10; N, 18.33%. IR (KBr, main peaks cm − 1): 2990 (w), 1526 (s), 1383 (m), 1268 (s), 936 (s). Compounds 2, 4, 5, and 6 were obtained by the same procedure as compound 1.
5. Supplementary material The full crystallographic details have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 163711, 163712 and 163713 for compounds 5, 3 and 1, respectively. Copies of the information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: + 44-1223-336-033; e-mail:
[email protected]. ac.uk or www: http://www.ccdc.cam.ac.uk).
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Table 1 Crystal and experimental data
Empirical formula Formula weight Wavelength (A, ) Crystal size (mm) Crystal morphology Crystal colour Crystal system Space group Z Unit cell dimensions a (A, ) b (A, ) c (A, ) h (°) i (°) k (°) V (A, 3) Dcalc F(000) q Range for data collection (°) Completeness (%) Independent reflections Observed reflections (I\2|(I)) Rint (I\2|(I)) Refined parameters R, wR2 (F 2) (I\2|(I)) wR2 (all) Goodness-of-fit on F 2 Largest difference peak and hole (e A, −3)
1
3
5
C22H36N12O12U 898.66 0.71073 0.20×0.20×0.15 cubic block blue monoclinic P21/c 2
C12.5H18ClN5O9U 655.8 0.71069 0.22×0.13×0.025 plate orange monoclinic C2/c 8
C24H32N8O12U 862.61 0.71069 0.20×0.10×0.10 block green triclinic P1( 1
10.934(2) 12.416(1) 12.56(2) 90.0 111.10(7) 90.0 1591(4) 1.875 880 2.00/29.94 99.8 4621 3107 0.019 286 0.018, 0.043 0.048 1.02 0.929/−0.880
27.689(2) 9.752(2) 16.048(2) 90.0 110.72(2) 90.0 4053(1) 2.149 2472 2.23/25.08 99.3 3575 2311 0.031 266 0.036, 0.063 0.074 0.99 0.839/−0.624
6.975(2) 9.651(2) 12.561(2) 76.74(1) 88.86(2) 73.14(2) 786.6(3) 1.821 420 2.27/29.95 98.3 4499 3674 0.032 205 0.038, 0.068 0.076 1.05 0.939/−1.775
Acknowledgements The TMR Research Network ERB 4061PL-97-0197 of the European Union, entitled ‘‘Molecular Magnetism: From Materials toward De6ices’’, is gratefully acknowledged for financial support.
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